The invention relates to a method and a radio station for transmitting data packets in a radio communication system, in particular in a mobile radio system.
In radio communication systems, information such as voice, image information, or other data, is transmitted with the aid of electromagnetic waves over a radio interface between a sending and a receiving radio station (base station and mobile station). Said waves are radiated using carrier frequencies lying in the frequency band provided for the respective system. Frequencies in the frequency band of approximately 2,000 MHz are provided for future mobile radio systems employing CDMA or TD/CDMA transmission methods over the radio interface, for example the UMTS (Universal Mobile Telecommunications System) or other 3rd generation systems. Frequency Division Multiplex Access (FDMA), Time Division Multiplex Access (TDMA), or a method known as Code Division Multiplex Access (CDMA) here serve to distinguish the signal sources.
As part of the process of specifying the UMTS standard, methods are currently being defined which are to be optimized for packet transmissions. An example of this is what is termed High Speed Downlink Packet Access (HSDPA) for the FDD (Frequency Division Duplex) and TDD (Time Division Duplex) mode of the UMTS standard. These methods are to be capable of catering for services with different Quality of Service (QoS) requirements. A base station (NB: Node B) is here responsible for what is termed the scheduling of different users' data and for what is termed the retransmission, which is to say the repeated transmission, of incorrectly received data packets using what is termed a hybrid ARQ process. Hybrid ARQ methods derive their advantage from the fact that incorrectly received data packets are stored in a storage facility in the receiving device so they can be combined with succeeding, repeated and, where applicable, modified transmissions of the data packets. In contrast to known pure ARQ methods where faulty data packets are rejected at the receiving side, this advantageously also makes it possible to benefit from the data packets already received to increase the quality of reception.
In the receiving device it is necessary for the received data to be forwarded to higher protocol layers in the correct sequence, which is to say in the sequence originally present at the sending side. Since, owing to retransmissions for instance, the time sequence of the correctly received data packets can become mixed-up during transmission over the radio interface, the original sequence has to be restored at the receiving side. Customarily implemented for this in the receiving device is a reordering buffer which continues to store a number of data packets until a correct sequence can be ensured. If, for instance, a second and a third data packet are correctly received while correct reception of a first data packet is still awaited, the two data packets will remain stored until the first data packet has also been correctly received. The data packets are then forwarded in the original sequence to higher protocol layers in the receiving device.
Because a base station potentially has to control and carry out transmissions of data packets to a large number of users in parallel, also referred to as ‘user scheduling’, long delays may disadvantageously arise until a repeatedly transmitted data packet has been correctly received. It may in such cases no longer be possible to maintain the pre-specified Quality of Service, and delays will occur that are unpleasant for the user on the receiving side.